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Article

Geochemical Features, Origin, and Depositional Environment of Late Ordovician–Early Silurian Wufeng and Longmaxi Formation Cherts in the Southeastern Sichuan Basin

by
Xiangying Ge
1,2,
Chuanlong Mou
1,2,3,*,
Xin Men
4,
Qiyu Wang
1,2,
Qian Hou
1,2,
Binsong Zheng
5 and
Feifei Chen
1,2
1
Chengdu Centre, China Geological Survey (Geosciences Innovation Center of Southwest China), Chengdu 610081, China
2
Key Laboratory of Sedimentary Basin and Oil and Gas Resources, Ministry of Natural Resources, Chengdu 610081, China
3
Shandong University of Science and Technology, Qingdao 266590, China
4
Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Ministry of Education), School of Geosciences and InfoPhysic, Central South University, Changsha 410083, China
5
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(8), 745; https://doi.org/10.3390/min14080745
Submission received: 22 June 2024 / Revised: 19 July 2024 / Accepted: 23 July 2024 / Published: 25 July 2024
(This article belongs to the Special Issue Environment and Geochemistry of Sediments, 2nd Edition)
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Abstract

:
The Sichuan Basin in Southeastern China contains extensive bedded cherts dating back to the Late Ordovician–Early Silurian period. To investigate the origin and depositional environment of these cherts, we conducted a comprehensive study using field observations, thin sections microscopic, silicon isotope analysis, and major and trace element geochemistry of samples from three sections. Our results indicate that the cherts from Wufeng and Longmaxi formations are non-hydrothermal, normal biogenic seawater deposits mainly affected by terrigenous input and slightly associated with volcanic ash. Al2O3/(Al2O3 + Fe2O3T) and LaN/CeN ratios, δCe values and Fe2O3T/TiO2 − Al2O3/(Al2O3 + Fe2O3T), Fe2O3T/(100 − SiO2) − Al2O3/(100 − SiO2), 100 × (Fe2O3T/SiO2) – 100 × (Al2O3/SiO2), and LaN/CeN − Al2O3/(Al2O3 + Fe2O3T) discrimination diagrams indicated that the bedded cherts deposited in the continental margin environment.

1. Introduction

Chert is typically defined as a siliceous rock with a silica content of more than 75–80 wt% [1]. It occurs in various forms, including nodules, concretions, bands, and bedded, and is often accompanied by black shales or carbonates [2]. Due to its density, high hardness, and resistance to weathering, chert has been extensively studied in various fields, including depositional environment, tectonic activity, paleontology, and paleogeography [3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The origin of chert is still a topic of ongoing research, with studies focusing on silica sources and silicon formation mechanisms. The silica sources include biogenic [11,17,18,19,20,21,22], hydrothermal [23,24], and terrestrial sources [3,4,23,25,26]. The formation mechanisms include biogenic [17], hydrothermal [3,4,23,27], metasomatic [28], volcanic genesis [29], and dissolution, reprecipitation, and diagenesis of air-borne dust [24,30]. In this study, we conducted a comprehensive investigation into the origin of cherts from the Wufeng and Longmaxi formations in South China. These formations are characterized by the presence of bedded cherts, which have been widely studied in the past. However, previous studies have often combined the cherts from both formations together [13,22,31,32,33,34,35,36], without considering the distinct geological events that occurred between them, such as volcanic action and Hirnantian glaciation, two events that are considered to have some direct and indirect relationships with the formation of the cherts. Our study aimed to further clarify the origin of these cherts by integrating field survey, petrological identification, Si isotope analysis, and major and trace element analysis of chert samples from each formation.

2. Geological Setting

The Sichuan Basin is part of the Upper Yangtze block in South China and has endured multiphase structural movement [37]. In the Middle-Late Ordovician, the tectonic compression intensified continuously in South China, affected by the Caledonian Movement (Figure 1A). The area of the Central Sichuan, Central Guizhou, and Xuefeng Uplift continuously increased, and the Sichuan Basin was submerged and turned into a back-bulge marine basin surrounded by these uplifts [37,38,39,40,41,42].
The Sichuan Basin deposited Wufeng, Guanyinqiao, and Longmaxi formations in the Late Ordovician–Early Silurian. The Wufeng and Longmaxi formations primarily contain black shales and bedded cherts (Figure 2A–C), interbedded with bentonites. Radiolaria fossils are observed in the bedded chert (Figure 2D–I). The Guanyinqiao Formation is characterized by thin-bedded limestones or mudstones with plentiful and diverse shelly fauna (Hirnantia fauna) [43,44]. We selected three outcrops (FCP, HYXP, DTBP) to analyze the origin of these cherts from the Wufeng and Longmaxi formations (Figure 1B). The Fucheng section (FCP, GPS: N 32°28′39″, E 107°14′11.4″) is located in Nanzheng district, Hanzhong city, Shanxi province, north of the Sichuan Basin. The Huangyingxiang section is (HYXP, GPS: N 29°12′48″, E 107°41′36″) located in Wulong district, Chongqing city, central Sichuan Basin. The Datianba section (DTBP, GPS: N 28°28′3.74″, E 108°55′54.1″) is located in Xiushan county, Chongqing province, southeast of the Sichuan Basin. Here, we use petrological and geochemical data of the bedded chert from the two formations to analyze the silica origins and depositional environment of the chert.

3. Samples and Methods

We chose 13, 28, and 16 bedded chert samples in the Huangyingxiang, Datianba, and Fucheng sections, respectively. Eight samples came from the Wufeng formation and 5 samples from the Longmaxi formation in the Huangyingxiang section; 22 samples came from the Wufeng formation and 6 samples from the Longmaxi formation in the Datianba section; and 7 samples came from the Wufeng formation and 9 samples from the Longmaxi formation in the Fucheng section. All cherts were airdried and powdered to 200 mesh. Major, rare earth, trace elements, and silicon isotopes were measured at the Beijing Research Institute of Uranium Geology, China. The major elements were measured by a Philips PW2404 X-ray fluorescence spectrometer (Philips Lighting Holding B.V., Eindhoven, Netherlands). The loss on ignition was documented as weight loss after burning at 1000 °C. The analytical uncertainty is generally <5%. Trace and rare earth elements were measured by an Element XR inductively coupled plasma mass spectrometer. Then, 50 mg rocks were digested by 1 mL of HF and 0.5 mL of HNO3 in a Teflon bomb at 185 ℃ for 24 h, dried, and then treated with HNO3 (0.5 mL). All the cherts received two acid digestions with HNO3 (5 mL) at 130 ℃ for 3 h. Dissolved samples were diluted to 50 mL in a clean PET bottle prior to analysis. The analytical uncertainty was usually <5%.
Sixty cherts were prepared for isotope analysis. The Si isotope ratios were analyzed with a MAT-253 gas-source isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). All the rocks were powdered to 200 mesh and were burned in a muffle furnace at 800 °C for 4 h, and the samples were placed in a vacuum oven and dried at 100 °C for half a day. Then, the samples were poured into the reactor and vacuumized. Bromine pentafluoride reagent was added and it was heated to 550~600 °C for a fluorination reaction of about 6~14 h in a heating furnace. SiF4 was extracted and purified repeatedly, and finally, the purified SiF4 samples were collected and sent to mass spectrometry. Silicon isotope data were normalized by the international quartz reference material NBS-28, and the δ30Si is obtained from the following equation: δ30Si (‰) = ([30Si/28Si]sample/[30Si/28Si]NBS-28–1) * 1000. The precision was better than ±0.1‰.

4. Geochemistry

4.1. Major Elements

Major element concentrations of cherts are listed in Table 1. The bedded cherts in the Wufeng formation have a SiO2 content ranging from 67.97% to 93.29% (avg. 79.60%), and Al2O3 content ranging from 2.33% to 14.95% (avg. 6.88%). The second most abundant oxides are CaO (0.052%–2.15%), the total Fe2O3 content Fe2O3T (0.404%–3.54%), and K2O (0.663%–4.62%). The concentrations of MgO, Na2O, P2O5, TiO2, and MnO in the cherts are 0.231%–1.56%, 0.094%–0.679%, 0.014%–0.126%, 0.106%–0.675%, and <0.004%–0.043%, respectively.
The bedded cherts in the Longmaxi formation are characterized by a dominance of SiO2 (63.80%–86.22%, avg. 75.47%), and Al2O3 follows SiO2 in abundance (4.12%–13.07%, avg. 8.31%). The contents of CaO range from 0.071% to 5.49%, with an average of 1.03%. The Fe2O3T (0.374%–3.98%, avg. 1.86%), Na2O (0.106%–1.5%, avg. 0.61%), MgO (0.401%–1.67%, avg. 0.84%), and K2O (1.15%–3.85%, avg. 2.34%) are slightly higher than those of the Wufeng formation cherts (Fe2O3T, avg. 1.75%, Na2O, avg.0.40%, MgO, avg. 0.73%, K2O, avg. 1.96%). The concentrations of MnO (<0.004%–0.031%), TiO2 (0.213%–0.735%), and P2O5 (0.023%–0.169%) are lower than 1%.

4.2. Rare Earth Elements

The values of the rare earth elements (REE) are listed in Table 2, displayed as post-Archean Australian shale (PAAS)-normalized patterns in Figure 3. The calculation formulas of the Eu and Ce anomalies are as follows:
δEu = Eu/Eu* = (EuN)/[(SmN×GdN)1/2], δCe = Ce/Ce* = 2CeN/(LaN + PrN), where the subscript N denotes the normalization of the REE to the PAAS [45].
All the cherts are normalized to PAAS and exhibit similar REE distribution, showing relatively flat, slight LREE enrichment with no notable Eu and Ce anomalies.
The total rare earth element contents (ΣREE) of the bedded cherts from the Wufeng formation vary from 46.61 to 336.51 ppm (avg. 136.91 ppm). The ratios of light to heavy REEs (LREE/HREE) range from 5.41 to 13.18, with an average of 9.03. The (LaN/YbN) ratios range from 0.57 to 1.45, with an average of 0.94. The Eu/Eu* and Ce/Ce* values exhibit slight negative Eu and Ce anomalies, and the Eu/Eu* and Ce/Ce* change from 0.80 to 1.16, with an average of 0.92 and 0.86 to 1.17, with an average of 0.92, respectively.
The total contents of rare earth elements (ΣREE) of the bedded cherts from the Longmaxi formation range from 70.02 to 232.84 ppm, with a median value of 153.17 ppm. The ratio of light to heavy REEs (LREE/HREE) ranges from 8.20 to 12.14, and the samples are slightly LREE-enriched. The (LaN/YbN) ratios fluctuate from 0.98 to 1.61, with an average of 1.16. The Ce/Ce* values show a few negative Ce anomalies, and the Ce/Ce* values change from 0.82 to 0.89, with an average of 0.87. The cherts display variably negative Eu anomalies, with Eu/Eu* values ranging from 0.72 to 1.04 (avg. 0.92).

4.3. Silicon Isotope Compositions

Sixty bedded chert samples from three sections (HYXP, DTBP, FCP) were analyzed for silicon isotopes (Table 3). The 30Si values of the samples in the Wufeng formation range from −0.5‰ to 0.5‰, with an average of 0.06‰. The 30Si values from the HYXP section change from −0.5‰ to 0.1‰, while the 30Si values from the DTBP section range from −0.2‰ to 0.3‰. The 30Si values of the nine samples in the Wufeng formation from the FCP section are 0.1‰ (2 samples), 0.2‰ (3 samples), 0.3‰(1 sample), 0.4‰ (2 samples), and 0.5‰ (1 sample). The 30Si values of the bedded chert samples from the Longmaxi formation vary from −0.5‰ to 0.3‰, with an average of −0.1‰. The 30Si values of the six samples in the Longmaxi formation from the HYXP section are −0.5‰ (2 samples), −0.2‰ (2 samples), −0.3‰ (1 sample), and −0.1‰(1 sample). The 30Si values of the six samples in the Longmaxi formation from the DTBP section are −0.1‰ (2 samples) and −0.2‰ (4 samples). The 30Si values of the nine samples in the Longmaxi formation from the FCP section are −0.2‰ (2 samples), 0.1‰ (4 samples), 0.2‰ (2 samples), and 0.3‰ (1 sample).

5. Discussion

5.1. Origin of the Cherts

Major elements, for instance, Fe, Mn, Al, and Ti, are important pointers to infer the origin of cherts. The concentration of Fe and Mn is primarily associated with the involvement of hydrothermal fluid, while the high contents of Al and Ti are in connection with multiple terrigenous inputs [2,3,46]. Adachi et al. [3] and Yamamoto [23] pointed out that the Al/(Al + Fe + Mn) ratio in neat chert associated with hydrothermal fluid is 0.01, but in connection with siliceous organisms, it is 0.6. In the Wufeng formation, the Al/(Al + Fe + Mn) ratios of the 37 chert samples range from 0.61 to 0.90, suggesting a non-hydrothermal origin. The Al/(Al + Fe + Mn) ratios of the 37 cherts from the Longmaxi formations range from 0.68 to 0.92, which also indicates that the silica originated from terrigenous input and siliceous organisms. Al-Fe-Mn ternary diagrams have been used to differentiate between hydrothermal and biogenic origins in cherts [3]. In the Al-Fe-Mn ternary diagram, all samples are near the Al–Fe line because of the low Mn contents (Figure 4A). All the cherts fall within, or adjacent to, the nonhydrothermal area. The Fe/Ti-Al/(Al + Fe + Mn) relationship diagram was developed based on the cherts found in modern ocean sediments of biological, hydrothermal, and terrigenous origins [46]. Seen from the Fe/Ti–Al/(Al + Fe + Mn) graph (Figure 4B), all the samples from the two formations are close to the terrigenous sediments area. The ratio of Al2O3/TiO2 is slightly influenced by deposition, digenesis, and weathering; therefore, it is a very useful indicator of the chert origin; Huang et al. [47] established the association graph of Al2O3/TiO2-Al/(Al + Fe + Mn). In the Al2O3/TiO2-Al/(Al + Fe + Mn) diagram (Figure 4C), the spread of the samples of the two formations is almost the same, which are plotted within or adjacent to the C region (the nonhydrothermal chert associated with the normal marine deposits).
REEs are also usually utilized to differentiate between hydrothermal and non-hydrothermal cherts [12,16,48,49,50]. Shimizu and Masuda [51] found that hydrothermal cherts showed negative Ce anomalies (avg. δCe = 0.29), whereas non-hydrothermal cherts exhibited positive Ce anomalies (avg. δCe = 1.2). Fleet [52] believed that hydrothermal cherts had low total REE content, negative Ce anomaly, and slight HREE enrichment, and that non-hydrothermal deposits have high ∑REE content, positive Ce anomaly, and no HREE enrichment. Based on previous studies, hydrothermal cherts are commonly featured by low∑REEs and LREEs, left-skewed REE distribution, negative Ce, and positive Eu anomalies by reason of siliceous biological activity deficiency [28,53,54,55,56], but in contrast, the chert affected by terrigenous input manifested high REEs and enrichment of light REEs without notable Ce anomaly [11,57]. In our study, ΣREE of the bedded cherts from the Wufeng formation vary from 46.61 to 336.51 ppm (avg. 136.91 ppm), with no notable Eu and Ce anomalies (0.80 ≤ δEu ≤ 1.16, avg. 0.92; 0.86 ≤ δCe ≤ 1.17, avg. 0.92), which is the characteristics of the non-hydrothermal cherts influenced by terrigenous input. ΣREE of the bedded cherts from Longmaxi formation vary from 70.02 to 232.84 ppm (avg. 153.17 ppm), slightly LREE-enriched with a weak negative Ce anomaly (0.82 ≤ δCe ≤ 0.89, avg. 0.87) and no notable Eu anomaly (0.72 ≤ δEu ≤ 1.04, avg. 0.92), which is also similar to the characteristics of the non-hydrothermal cherts mainly influenced by terrigenous input.

5.2. Silicon Isotopes

Si isotope composition reflects the origins of the cherts, as well [58,59]. Biological activities in shallow marine could especially remove δ28Si and contribute to the content of δ30Si in the sea [60,61]. Ding et al. [62] indicated that hydrothermal quartz has a small δ30Si value (−1.5‰ to 0.8‰); authigenic quartz in low-temperature water has a large δ30Si value (1.1‰ to 1.4‰); secondary diagenetic quartz falls between the δ30Si ranges of hydrothermal and authigenic quartz (0.8‰ to 1.1‰); and the δ30Si values of metasomatic chert vary from 2.4 ‰ to 3.4‰. The average δ30Si values of abyssal chert ranged from −0.6‰ to 0.8‰, and that of semi-abyssal and offshore shallow sea chert varied from 0.3‰ to 1.3 [27,58,63]. Ding et al. [62] studied Chinese cherts from different periods and concluded that the δ30Si values are mainly enriched in two ranges. The first range is from 0.1 to −0.5‰, which is in accordance with the δ30Si range of volcanic and abyssal radiolarian cherts. The second is from 0.3 to 1.3%, which is similar to the δ30Si range of shallow-marine and semi-abyssal radiolarian chert. The δ30Si values of the samples in the Wufeng formation (−0.5‰ to 0.5‰, avg. 0.06‰) and Longmaxi formation (−0.5‰ to 0.3‰, with an average of −0.1‰.) are identical with those mentioned for seawater cherts, suggesting that the origin of these cherts is dominantly from organisms in seawater and slightly influenced by the volcanic ash.
Van den Boorn et al. [64] established an Al2O3-δ30Si diagram and figured out three end-member modes of chert formation. One end-member is an Al2O3-rich chert which has a δ30Si value of approximately, or slightly below, zero, indicating an appreciable quantity of silica from precursor volcaniclastic sediments or crystalline rocks. Two other end-members are Al2O3-poor, high-δ30Si silica deposited from seawater and Al2O3-poor, high-δ30Si silica precipitated from hydrothermal fluid. Almost all the cherts of the Wufeng and Longmaxi formations have high Al2O3 and δ30Si values close to, or slightly below, zero, reflecting a substantial amount of silica from volcanic ashes (Figure 5).

5.3. Depositional Environment

Previous research has indicated that some ratios of major elements and REEs in cherts are commonly used to assess the depositional environments [2,12]. Murray [2] showed that the ratio of Al2O3/(Al2O3 + Fe2O3T) fluctuated from 0.5 to 0.9 in the continental margin, less than 0.4 in the mid-ocean ridge, and varied from 0.4 to 0.7 in the pelagic environment. It is generally believed that when the chert sedimented near the continental margin and was affected by terrigenous debris, Al2O3/(Al2O3 + Fe2O3T) was more than 0.5 and Fe2O3/TiO2 was less than 50; and that when the chert deposited close to the mid-ocean ridge and was affected by the hydrothermal, Al2O3/ (Al2O3 + Fe2O3T) was less than 0.5 and Fe2O3/TiO2 was more than 50 [12]. Also, δCe in the chert is influenced by water, terrigenous input, and the deposition rate [2,65]; δCe ranged from 0.18 to 0.6 (avg. 0.29) deposited close to the mid-oceanic ridge, between 0.5–0.76 (avg. 0.6) near the pelagic environment, and from 0.67 to 1.52 (avg. 1.11) near the continental margin [2]. LaN/CeN is also an effective index for the depositional environment of chert [2]. The LaN/CeN of the chert deposited near the ridge is more than 3.5, near the pelagic ocean basin from 2 to 3, and near the continental margin approximately equal to 1.
The Al2O3/(Al2O3 + Fe2O3T) ratio of the cherts ranged from 0.68 to 0.92 (Wufeng formation) and from 0.74 to 0.94 (Longmaxi formation), respectively, which are near the continental margin range chert (0.5–0.9). The average δCe values of the cherts in the Wufeng and Longmaxi (0.86 ≤ δCe ≤ 1.17, avg. 0.92; 0.82 ≤ δCe ≤ 0.89, avg. 0.87) were similar to the chert from the continental margin of Monterey Formation (0.91 ± 0.06) [65]. LaN/CeN values of the bedded chert in the Wufeng and Longmaxi formations varied from 0.41to 0.61, with an average of 0.54, and 0.51 to 0.60, with an average of 0.56, respectively, which were approximate to the chert deposited in the continental margin (LaN/CeN ≈ 1). Murray [2] and Adachi et al. [3] developed identification diagrams, such as Fe2O3T/TiO2 − Al2O3/(Al2O3 + Fe2O3T), Fe2O3T/(100 − SiO2) − Al2O3/(100 − SiO2), 100 × (Fe2O3T/SiO2) – 100 × (Al2O3/SiO2), and LaN/CeN − Al2O3/(Al2O3 + Fe2O3T), built on the geochemical data of the chert in different depositional environments from the Early Paleozoic to the Tertiary era. In the Fe2O3T/TiO2 − Al2O3/(Al2O3 + Fe2O3T), Fe2O3T/(100 − SiO2) − Al2O3/(100 − SiO2), and LaN/CeN − Al2O3/(Al2O3 + Fe2O3T) diagrams (Figure 6A,C,D), all samples from the two formations plotted near to, or within, the continental margin field. However, in the 100 × (Fe2O3T/SiO2) – 100 × (Al2O3/SiO2) diagram (Figure 6B) for the Wufeng formation, most of the samples deposited near to, or within, the continental margin, with the exception of FCP-B8. For the Longmaxi formation, although all the samples are plotted outside of the continental margin field, most of the samples were near to this region, with the exceptions of FCP-B25 and DTBP-B25-B30.
In sum, according to the comprehensive analysis of the major rare earth elements and these diagrams, we conclude that the bedded chert in the Wufeng and Longmaxi formations formed in the continental margin environment.

6. Conclusions

In this study, on the basis of the petrologic features and detailed geochemical analyses of the cherts from the Wufeng and Longmaxi formations, we attempt to ascertain the origin and depositional environment of the cherts in the two formations.
Al/(Al + Fe +Mn) ratios, the Al–Fe–Mn ternary diagram; Fe/Ti-Al/(Al + Fe +Mn), Al2O3/TiO2-Al/(Al + Fe + Mn) and Al2O3-δ30Si diagrams; REEs (δCe and δEu); thin section analysis reflected that the chert in the Wufeng and Longmaxi formations is non-hydrothermal, normal biogenic seawater deposits primarily affected by terrigenous input, and slightly associated with volcanic ash. Moreover, during the Ordovician–Silurian transition, frequent volcanisms brought a lot of tephra, which provided lots of nutrients, increased ocean primary productivity, and promoted the blooming of siliceous organisms. Al2O3/(Al2O3 + Fe2O3T) and LaN/CeN ratios, δCe values and Fe2O3T/TiO2 − Al2O3/(Al2O3 + Fe2O3T), Fe2O3T/(100 − SiO2) − Al2O3/(100 − SiO2), 100 × (Fe2O3T/SiO2) – 100 × (Al2O3/SiO2) and LaN/CeN − Al2O3/(Al2O3 + Fe2O3T) discrimination diagrams indicated that the cherts from the Wufeng and Longmaxi formations both precipitated in the continental margin.

Author Contributions

Conceptualization, X.G. and X.M.; methodology, X.G. and Q.W.; investigation, X.G., X.M., B.Z. and Q.H.; resources, X.G.; data curation, F.C.; writing—original draft preparation, X.G.; writing—review and editing, C.M.; visualization, X.G.; supervision, C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by China Geological Survey Projects (DD20242003; DD20242564; DD20242114) and the National Natural Science Foundation of China(U2344209).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We thank other colleagues in the Chengdu Center, China Geological Survey (Geosciences Innovation Center of Southwest China) for the fruitful discussions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) The Late Ordovician–Early Silurian paleogeographic map of the world (following [20]). (B) The study area and locations of the sections.
Figure 1. (A) The Late Ordovician–Early Silurian paleogeographic map of the world (following [20]). (B) The study area and locations of the sections.
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Figure 2. Field photos and microscopic characteristics of the bedded cherts in the Wufeng and Longmaxi formations of the Sichuan Basin. (A) The bedded cherts of the Wufeng formation in the Fucheng section. (B) The bedded cherts of Longmaxi formation in the Huangyingxiang section. (C) The bedded chert of Wufeng formation in the Datianba section. (D,E) Radiolaria developed in the cherts of Wufeng (D) and Longmaxi (E) formations. (F–H) Radiolaria developed in the cherts of Wufeng (F) and Longmaxi (G,H) formations in the Huangyingxiang section. (I) Radiolaria developed in the cherts of the Wufeng formation in the Datianba section. The yellow arrows indicate the Radiolaria.
Figure 2. Field photos and microscopic characteristics of the bedded cherts in the Wufeng and Longmaxi formations of the Sichuan Basin. (A) The bedded cherts of the Wufeng formation in the Fucheng section. (B) The bedded cherts of Longmaxi formation in the Huangyingxiang section. (C) The bedded chert of Wufeng formation in the Datianba section. (D,E) Radiolaria developed in the cherts of Wufeng (D) and Longmaxi (E) formations. (F–H) Radiolaria developed in the cherts of Wufeng (F) and Longmaxi (G,H) formations in the Huangyingxiang section. (I) Radiolaria developed in the cherts of the Wufeng formation in the Datianba section. The yellow arrows indicate the Radiolaria.
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Figure 3. PAAS-normalized REE distribution patterns of the cherts from the Wufeng and Longmaxi formations in HYXP, DTBP, and FCP sections of the Sichuan Basin (normalization values after [45]).
Figure 3. PAAS-normalized REE distribution patterns of the cherts from the Wufeng and Longmaxi formations in HYXP, DTBP, and FCP sections of the Sichuan Basin (normalization values after [45]).
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Figure 4. Origin of the bedded cherts from the Wufeng and Longmaxi formations of three sections (HYXP, DTBP, FCP) in the Sichuan Basin. (A) Ternary diagram of Al-Fe-Mn of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [3]). (B) Fe/Ti-Al/(Al + Fe + Mn) relationship diagram of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [46]). (C) Al2O3/TiO2-Al/(Al + Fe + Mn) diagram of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [47]). A: hydrothermal cherts associated with basaltic volcanism; B: non-hydrothermal cherts containing felsic volcanic clasts; C: non-hydrothermal cherts associated with the normal marine deposits; D: non-hydrothermal cherts containing basaltic volcanic clasts.
Figure 4. Origin of the bedded cherts from the Wufeng and Longmaxi formations of three sections (HYXP, DTBP, FCP) in the Sichuan Basin. (A) Ternary diagram of Al-Fe-Mn of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [3]). (B) Fe/Ti-Al/(Al + Fe + Mn) relationship diagram of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [46]). (C) Al2O3/TiO2-Al/(Al + Fe + Mn) diagram of the bedded cherts from the Wufeng and Longmaxi formations in three sections (following [47]). A: hydrothermal cherts associated with basaltic volcanism; B: non-hydrothermal cherts containing felsic volcanic clasts; C: non-hydrothermal cherts associated with the normal marine deposits; D: non-hydrothermal cherts containing basaltic volcanic clasts.
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Figure 5. Correlation of δ30Si and Al2O3 concentrations of the cherts from the Wufeng and Longmaxi formations of three sections (following [64]).
Figure 5. Correlation of δ30Si and Al2O3 concentrations of the cherts from the Wufeng and Longmaxi formations of three sections (following [64]).
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Figure 6. Discrimination diagrams of the depositional environment for the cherts from the Wufeng and Longmaxi formations of three sections in the Sichuan Basin. (A) LaN/CeN versus Al2O3/(Al2O3 + Fe2O3T) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]). (B) Fe2O3T/(100 − SiO2) versus Al2O3/(100 − SiO2) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [3]). (C) Fe2O3T/TiO2 versus Al2O3/(Al2O3 + Fe2O3T) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]). (D) 100 × (Fe2O3T/SiO2) versus 100 × (Al2O3/SiO2) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]).
Figure 6. Discrimination diagrams of the depositional environment for the cherts from the Wufeng and Longmaxi formations of three sections in the Sichuan Basin. (A) LaN/CeN versus Al2O3/(Al2O3 + Fe2O3T) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]). (B) Fe2O3T/(100 − SiO2) versus Al2O3/(100 − SiO2) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [3]). (C) Fe2O3T/TiO2 versus Al2O3/(Al2O3 + Fe2O3T) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]). (D) 100 × (Fe2O3T/SiO2) versus 100 × (Al2O3/SiO2) bivariate diagram for the cherts from the Wufeng and Longmaxi formations of three sections (following [2]).
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Table 1. Analytical data of major elements(wt%) of the cherts from the Wufeng and Longmaxi formations.
Table 1. Analytical data of major elements(wt%) of the cherts from the Wufeng and Longmaxi formations.
SampleFm.SiO2Al2O3CaOFe2O3TK2OMgOMnONa2OP2O5TiO2LOIAl/(Al + Fe + Mn)Al2O3/(Al2O3 + Fe2O3T)Fe/Ti
HYXP-B6WF85.075.110.0640.4251.490.415<0.0040.1260.0250.2716.910.900.921.83
HYXP-B776.268.730.0781.112.520.699<0.0040.4210.0310.3999.710.860.893.25
HYXP-B880.916.920.0850.8731.950.543<0.0040.4580.0290.3477.840.860.892.94
HYXP-B984.954.130.0850.8051.150.314<0.0040.3240.0180.2137.480.800.844.41
HYXP-B1084.084.590.0730.4981.280.341<0.0040.3480.0190.2798.490.870.902.08
HYXP-B1187.33.930.1440.4041.080.29<0.0040.3170.0140.2056.290.880.912.30
HYXP-B1285.173.900.1190.6071.080.278<0.0040.3290.0180.227.910.830.873.22
HYXP-B1374.197.830.6291.692.230.717<0.0040.5430.0360.45211.30.780.824.36
HYXP-B17LM76.47.510.2011.682.190.571<0.0040.6320.0540.40410.340.770.824.85
HYXP-B1876.670.6241.720.545<0.0040.6060.0780.36310.010.760.805.46
HYXP-B1974.488.670.3431.712.490.712<0.0040.7170.0850.4610.320.790.844.34
HYXP-B2080.585.640.1961.21.60.441<0.0040.4890.0480.3049.030.780.824.61
HYXP-B2178.266.70.1621.791.90.518<0.0040.560.0870.3619.10.740.795.78
DTBP-B1WF79.37.830.1131.552.140.80.0040.5690.0690.426.670.790.834.31
DTBP-B277.638.680.1151.822.510.714<0.0040.6790.090.4837.220.780.834.40
DTBP-B378.947.550.1132.122.080.7180.0040.570.0760.4056.840.730.786.11
DTBP-B4778.440.2762.612.360.9310.0140.6250.1080.426.780.710.767.25
DTBP-B577.87.150.5342.991.990.9640.0180.5680.0840.3787.060.640.719.23
DTBP-B677.726.281.072.951.751.120.0210.5010.0780.3357.620.620.6810.27
DTBP-B778.467.010.1482.451.930.7930.0060.5180.0850.3687.690.680.747.77
DTBP-B877.457.360.4572.722.030.9070.0110.5510.0750.3867.570.670.738.22
DTBP-B979.195.711.392.511.581.030.0190.4520.0660.3187.350.630.699.21
DTBP-B1076.186.122.152.781.711.560.0430.4890.080.3148.480.620.6910.33
DTBP-B1181.355.591.011.991.560.970.0170.450.0750.2926.630.680.747.95
DTBP-B1280.95.40.9792.61.470.9530.0370.4360.0760.2876.840.610.6810.57
DTBP-B1379.746.590.3822.111.810.8340.0080.4990.0950.3586.990.700.766.88
DTBP-B1477.827.610.2892.632.10.9210.0060.5360.1120.4417.410.690.746.96
DTBP-B1576.257.980.2133.542.20.8790.0070.5590.1260.447.710.630.699.39
DTBP-B1679.697.440.1091.822.130.6110.0040.480.0630.4257.190.760.805.00
DTBP-B1780.417.150.1211.782.110.566<0.0040.4580.0790.4216.360.750.800.00
DTBP-B1878.987.690.1411.722.280.6390.0040.4660.0630.45970.770.824.37
DTBP-B1975.919.130.2762.472.580.8860.0050.6130.1210.4977.160.740.795.80
DTBP-B20807.250.1061.582.130.578<0.0040.4880.0910.4327.30.780.824.27
DTBP-B2182.056.190.2490.9831.790.50.0040.3480.050.3687.370.830.863.12
DTBP-B2278.087.610.2722.12.110.660.0040.4350.0720.448.170.730.785.57
DTBP-B25LM68.7113.070.3272.773.850.9740.0051.50.1160.7357.580.780.834.40
DTBP-B2670.9811.820.3432.273.520.9120.0041.210.0950.7048.090.800.843.76
DTBP-B2764.8412.132.943.983.491.550.0191.310.1690.6958.840.700.756.68
DTBP-B2865.2710.834.73.43.051.630.0271.010.1430.6229.20.700.766.38
DTBP-B2965.3310.94.333.663.111.670.031.060.1380.6229.120.690.756.86
DTBP-B3063.810.595.493.7531.630.0311.060.1520.6249.850.680.747.01
FCP-B5WF67.9714.950.0551.824.621.45<0.0040.1050.0580.6757.870.860.893.15
FCP-B876.0110.780.0781.333.381.05<0.0040.0950.0520.5836.630.860.892.66
FCP -B983.276.830.1130.90320.667<0.0040.1220.0550.3055.290.850.883.45
FCP -B1084.75.620.0521.51.650.525<0.0040.1170.050.2735.420.740.796.41
NFP-B1293.292.330.0580.620.6630.231<0.0040.0940.0270.1062.170.740.796.82
FCP-B1472.937.60.2920.762.240.72<0.0040.0970.0330.42114.880.880.912.11
FCP-B1578.133.620.2011.470.9530.348<0.0040.1770.0590.2114.810.650.718.17
FCP-B18LM86.224.120.190.7341.150.401<0.0040.1060.0240.2136.770.810.854.02
FCP-B1979.46.460.080.4221.70.562<0.0040.2130.0230.3310.720.920.941.49
FCP-B2077.17.650.0820.7852.140.704<0.0040.1710.0240.3910.910.880.912.35
FCP-B2183.355.540.1320.721.350.46<0.0040.3220.040.3147.680.850.882.68
FCP-B2284.624.750.150.3741.230.429<0.0040.1750.0320.2847.410.910.931.54
FCP-B2382.576.90.0740.4671.90.649<0.0040.1850.0420.3456.860.920.941.58
FCP-B2482.416.810.0710.511.90.643<0.0040.2480.0360.3546.990.910.931.68
FCP-B2571.4710.550.13.792.91<0.0040.390.0770.5469.160.680.748.10
FCP-B2677.018.640.0841.432.390.822<0.0040.2540.0470.4688.820.820.863.56
Table 2. Analytical data of rare earth elements (REE) of the cherts from the Wufeng and Longmaxi formations.
Table 2. Analytical data of rare earth elements (REE) of the cherts from the Wufeng and Longmaxi formations.
SectionHYXPDTBP
FM.WFLMWF
SampleB6B7B8B9B10B11B12B13B17B18B19B20B21B1B2
La21.948.328.21621.916.217.73029.626.632.720.725.128.639.3
Ce43.68846.528.336.628.330.952.751.545.555.834.841.949.664.7
Pr5.9910.95.343.434.223.453.796.226.215.566.744.194.986.017.34
Nd2541.318.812.815.412.614.623.422.720.824.815.818.222.626.3
Sm4.826.732.872.012.542.012.313.733.53.283.932.612.973.854.54
Eu0.8971.10.5380.3850.4720.380.4280.6320.6670.5890.7360.5180.5610.6660.926
Gd4.074.762.371.652.171.61.753.082.952.633.192.072.393.074.04
Tb0.6770.6920.4040.2750.380.2510.2760.5180.4810.4180.5090.3480.3990.5490.779
Dy3.483.372.391.492.121.351.522.822.622.212.821.842.292.974.49
Ho0.6830.6930.5290.3190.4540.2930.3170.5870.5480.4590.5720.3760.470.5910.963
Er2.112.181.730.9871.390.9080.9491.781.731.431.851.181.51.822.9
Tm0.3770.4110.3360.1880.2610.1650.1690.3290.3050.2560.3310.2130.2780.3230.526
Yb2.532.752.261.161.651.111.122.122.011.622.171.41.882.193.39
Lu0.3970.4170.3480.1750.2430.1650.150.3070.2980.240.320.2260.2770.3290.506
∑REE116.53211.60112.6269.1789.868.7875.98128.22125.12111.59136.4786.27103.20123.17160.70
LREE/HREE7.1412.859.8610.089.3610.7711.1510.1110.4311.0510.6010.279.889.408.13
LaN/YbN0.641.290.921.020.981.081.161.041.091.211.111.090.980.960.85
Eu/Eu*0.940.910.960.990.940.990.990.870.970.940.971.040.980.901.01
Ce/Ce*0.88 0.88 0.87 0.88 0.88 0.87 0.87 0.89 0.870.860.860.860.860.870.88
SectionDTBP
FM. WF
SampleB3B4B5B6B7B8B9B10B11B12B13B14B15B16B17
La29.744.527.423.927.228.523.22523.421.325.830.330.727.230
Ce51.687.749.442.650.451.441.143.641.538.146.555.955.549.353.2
Pr6.34116.195.266.556.35.065.335.034.765.867.026.855.926.37
Nd24.343.424.320.627.524.119.920.919.518.822.928.127.322.623.7
Sm4.548.344.544.035.74.493.913.953.723.674.425.555.333.814.01
Eu0.7781.340.8310.791.110.8180.7470.730.750.720.8461.031.040.6790.727
Gd3.986.834.023.745.523.843.493.663.343.364.074.844.893.033.36
Tb0.7011.10.6940.6560.960.6640.6040.6480.5880.5880.6830.8420.8360.5190.565
Dy3.775.623.643.55.033.653.183.43.123.163.664.454.512.863.04
Ho0.7631.10.7190.6950.9730.7310.6210.6710.5830.6280.7280.8810.8990.5850.631
Er2.263.112.0822.632.121.741.911.731.722.082.52.511.741.84
Tm0.4040.5320.3680.350.4510.3710.2980.3390.3010.2870.3540.4430.4420.3210.338
Yb2.583.362.292.262.772.521.892.031.911.812.232.72.682.112.2
Lu0.3780.4880.3440.3150.3980.3680.2680.2940.2680.2650.330.3910.4010.3060.321
∑REE132.09 218.42 126.82 110.70 137.19 129.87 106.01 112.46 105.74 99.17 120.46 144.95 143.89 120.98 130.30
LREE/HREE7.90 8.87 7.96 7.19 6.32 8.10 7.77 7.68 7.93 7.39 7.52 7.50 7.38 9.55 9.60
LaN/YbN0.85 0.98 0.88 0.78 0.72 0.83 0.90 0.91 0.90 0.87 0.85 0.83 0.84 0.95 1.00
Eu/Eu*0.85 0.83 0.91 0.95 0.92 0.92 0.94 0.90 0.99 0.96 0.93 0.93 0.95 0.93 0.92
Ce/Ce*0.86 0.91 0.87 0.87 0.87 0.88 0.87 0.87 0.88 0.87 0.87 0.88 0.88 0.89 0.88
SectionDTBPFCP
FM.WFLMWF
SampleB18B19B20B21B22B25B26B27B28B29B30B5B8B9B10
La32.536.630.325.226.651.3455246.748.444.666.767.647.629.9
Ce5764.353.244.00 45.60 92.60 80.20 93.20 83.90 89.40 79.90 161.00 155.00 114.00 66.40
Pr6.637.616.265.115.3411.19.6411.410.311.19.881514.5126.93
Nd2528.523.218.719.440.635.843.739.741.738.454.954.150.127.6
Sm4.315.174.013.073.196.725.977.987.197.556.989.179.079.425.04
Eu0.6980.8370.7280.590.60.9150.9821.451.311.331.261.581.351.640.848
Gd3.794.623.472.642.735.294.76.966.186.175.936.946.716.993.88
Tb0.6930.8450.5960.4450.4870.8480.771.161.071.051.031.141.050.9620.585
Dy3.854.623.412.46 2.69 4.37 3.98 5.89 5.54 5.36 5.35 6.38 5.38 4.28 2.62
Ho0.7940.910.6980.52 0.57 0.86 0.79 1.16 1.05 1.03 1.01 1.40 1.06 0.77 0.49
Er2.342.722.121.54 1.72 2.44 2.35 3.24 2.99 2.92 2.83 4.52 3.27 2.29 1.41
Tm0.4140.4810.4070.2720.3140.4390.4450.5640.5090.5120.50.9110.6310.4480.265
Yb2.733.162.551.811.992.772.833.613.173.093.115.964.192.881.52
Lu0.4040.4580.360.2620.2920.4210.4110.5280.4640.4490.4480.9060.5970.4160.225
∑REE141.15 160.83 131.31 106.62 111.52 220.67 193.87 232.84 210.07 220.06 201.23 336.51 324.51 253.80 147.72
LREE/HREE8.40 8.03 8.65 9.72 9.33 11.65 10.91 9.07 9.02 9.69 8.96 10.95 13.18 12.33 12.43
LaN/YbN0.88 0.85 0.88 1.03 0.98 1.36 1.17 1.06 1.09 1.15 1.06 0.82 1.19 1.22 1.45
Eu/Eu*0.81 0.80 0.91 0.97 0.95 0.72 0.86 0.91 0.92 0.91 0.91 0.92 0.81 0.94 0.89
Ce/Ce*0.890.890.890.890.880.890.890.880.880.890.871.171.141.101.06
SectionFCP
FM.WFLM
SampleB12B14B15B18B19B20B21B22B23B24B25B26
La9.531.217.416.330.831.625.626.334.128.541.233.4
Ce20.568.433.927.354.754.244.646.167.156.274.862.2
Pr2.178.294.273.556.866.35.55.639.067.369.427.7
Nd8.4135.218.213.626.122.321.321.639.131.237.229.7
Sm1.576.513.372.314.343.443.723.557.956.17.335.4
Eu0.3471.120.6660.3890.7240.6330.6450.6261.371.061.30.981
Gd1.236.133.931.933.262.653.092.766.24.736.554.28
Tb0.1880.9790.6890.3120.50.4440.50.4210.9470.7371.050.698
Dy1.045.333.81.582.562.392.612.264.073.415.393.95
Ho0.1881.060.7860.3280.4880.4980.50.4060.6840.5761.010.723
Er0.5533.082.220.9411.491.591.421.141.891.582.832.15
Tm0.1110.5380.3780.170.2670.3060.2530.1910.3410.2810.5080.385
Yb0.6983.362.231.141.711.971.561.22.121.73.112.52
Lu0.1040.4970.340.1650.2480.3040.2190.1720.2980.2410.4420.358
∑REE46.61 171.69 92.18 70.02 134.05 128.63 111.52 112.36 175.23 143.68 192.14 154.45
LREE/HREE10.33 7.19 5.41 9.66 11.74 11.67 9.98 12.14 9.59 9.84 8.20 9.25
LaN/YbN1.00 0.68 0.57 1.05 1.33 1.18 1.21 1.61 1.19 1.24 0.98 0.98
Eu/Eu*1.16 0.83 0.85 0.86 0.90 0.98 0.89 0.93 0.91 0.92 0.88 0.95
Ce/Ce*1.040.980.900.820.870.880.860.870.880.890.870.89
Table 3. The silicon isotopes of bedded cherts from the Wufeng and Longmaxi formations.
Table 3. The silicon isotopes of bedded cherts from the Wufeng and Longmaxi formations.
SamplesFm.δ30SiV-NBS28‰SamplesFm.δ30SiV-NBS28‰
HYXP-B6WF−0.5DTBP-B17WF0.1
HYXP-B70.1DTBP-B180.1
HYXP-B8−0.1DTBP-B190.1
HYXP-B9−0.2DTBP-B200.1
HYXP-B10−0.4DTBP-B210.1
HYXP-B11−0.3DTBP-B220.3
HYXP-B12−0.1DTBP-B25LM−0.1
HYXP-B13−0.3DTBP-B26−0.2
HYXP-B16LM−0.5DTBP-B27−0.1
HYXP-B17−0.5DTBP-B28−0.2
HYXP-B18−0.2DTBP-B29−0.2
HYXP-B19−0.3DTBP-B30−0.2
HYXP-B20−0.1NFP-B5WF0.2
HYXP-B21−0.2NFP-B80.2
DTBP-B1WF−0.1NFP-B90.5
DTBP-B2−0.2NFP-B100.3
DTBP-B3−0.1NFP-B110.4
DTBP-B40.1NFP-B120.4
DTBP-B50.1NFP-B130.1
DTBP-B60.2NFP-B140.1
DTBP-B70.1NFP-B150.2
DTBP-B80.1NFP-B18LM0.1
DTBP-B90.3NFP-B19−0.2
DTBP-B10−0.1NFP-B200.2
DTBP-B11−0.1NFP-B210.2
DTBP-B12−0.1NFP-B220.1
DTBP-B130.3NFP-B230.1
DTBP-B140.2NFP-B240.3
DTBP-B150.1NFP-B25−0.2
DTBP-B160.2NFP-B260.1
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Ge, X.; Mou, C.; Men, X.; Wang, Q.; Hou, Q.; Zheng, B.; Chen, F. Geochemical Features, Origin, and Depositional Environment of Late Ordovician–Early Silurian Wufeng and Longmaxi Formation Cherts in the Southeastern Sichuan Basin. Minerals 2024, 14, 745. https://doi.org/10.3390/min14080745

AMA Style

Ge X, Mou C, Men X, Wang Q, Hou Q, Zheng B, Chen F. Geochemical Features, Origin, and Depositional Environment of Late Ordovician–Early Silurian Wufeng and Longmaxi Formation Cherts in the Southeastern Sichuan Basin. Minerals. 2024; 14(8):745. https://doi.org/10.3390/min14080745

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Ge, Xiangying, Chuanlong Mou, Xin Men, Qiyu Wang, Qian Hou, Binsong Zheng, and Feifei Chen. 2024. "Geochemical Features, Origin, and Depositional Environment of Late Ordovician–Early Silurian Wufeng and Longmaxi Formation Cherts in the Southeastern Sichuan Basin" Minerals 14, no. 8: 745. https://doi.org/10.3390/min14080745

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